Field of the Invention
The present invention is related to isocyanate modified epoxy resins containing core shell rubber particles for fusion-bonded epoxy coating applications and to powder coating compositions which comprise these resins. The compositions may be suitable, among other things, for making corrosion protection Fusion-Bonded Epoxy (FBE) coatings or primers for pipelines through which hydrocarbons are transported from production facilities to processing facilities at high temperatures (>110° C.).
Background of the Invention
As the service temperatures of oil and gas pipelines increase due to the exploitation of ultra deep reservoirs and tar sands, the pipe coating industry has been developing high performance corrosion protection coatings and insulating multilayer systems to meet the industry needs. Currently, the pipe coating industry can provide cost-effective FBE coatings systems to meet the requirements for corrosion protection of pipelines operating at temperatures of up to about 140° C. However, it has been predicted that the next generation of high service temperature pipelines will operate at even higher temperatures. To meet this requirement, the pipe coating industry needs FBE coating or primers systems that are able to protect from corrosion pipelines operating at higher service temperatures. Further, in order to be cost competitive the FBE coating or primer systems must be applicable by using state-of-the-art FBE powder coating technology.
A key ingredient in an FBE coating composition is the Solid Epoxy Resin (SER), which determines the properties of the coating to a high extent. One strongly desirable property of an SER for use in FBE coating applications is to provide good flexibility to the coating at subzero temperatures as measured for instance by the NACE RP0394-2002 H4.3 in degrees per pipe diameter (°/PD).
Once formulated into a powder coating composition, the SER also needs to impart a good balance of physical properties to the FBE coating. One important property of the finished FBE coating is its onset glass transition temperature that should be at least 5° C. above maximum pipeline design temperature according to ISO 21809-2 Table 2—‘Minimum Requirements for Coating Qualification Testing.’
However, higher glass transition temperature (Tg) does not generally lead to good flexibility; i.e. typically, the higher the Tg the lower the coating flexibility. Higher Tg is usually achieved by increasing the crosslink density of the thermoset polymer, however, this approach reduces coating flexibility. Consequently, the challenge is to break the Tg-flexibility paradigm without compromising other key coating requirements like storage stability, processability, adhesion, and corrosion resistance. In addition, the cured FBE coating must have adequate thermal aging to maintain its integrity for the lifetime of the pipeline.
Although the use of core-shell rubber particles has been known to toughen epoxy systems, it has now unexpectedly been found that by using a particular type of core shell rubber dispersed in a solid epoxy resin made from one or more polyisocyanate compounds for making an isocyanate modified epoxy resin, it is possible to produce a SER that, when incorporated into a powder coating composition, is capable of providing a tougher FBE coating that has an onset glass transition temperature of at least about 160° C. and a flexibility of at least 3°/PD and improved adhesion to the metal substrate.